The incidence and typical symptoms of schizophrenia are similar in diverse human populations; it is likely that the same is true for manic-depressive illnesses. Relative constancy of incidence over time and place (referred to as the 'anthropo-parity principle') and absence of evidence for environmental causation suggest that these disorders are genetic in origin and in some way characteristic of the human condition. Continued high prevalence in the face of a fertility disadvantage requires explanation--why do the genes persist and what is their function? It is proposed that the origins of psychosis are closely linked to the evolution of the human brain. The capacity for language evolved by the process of hemispheric specialisation, a gene for asymmetry (the 'right shift factor' or cerebral dominance gene) playing a critical role. This gene, it is suggested, is represented in homologous form on the X and Y chromosomes and has been subject to sexual selection. Such a locus could explain sex differences in cerebral asymmetry, and in age of onset and outcome of psychosis.